Certain laser systems employ acousto-optic devices to modulate polarized light beams in a variety of applications such as laser Q-switching, external modulation, and beam deflection. The following background information is presented herein only by way of example with reference to an acousto-optic device functioning as a Q-switch for a laser.
A conventional laser Q-switch includes an optically transparent acousto-optic medium through which incident light having a predetermined nonrandom polarization propagates along an optical path. An acoustic wave transducer coupled to the medium generates acoustic waves that propagate through the medium in a direction transverse to the optical path. The acoustic waves modulate the index of refraction of the medium and form a type of diffraction grating that diffracts the incident light. Detailed descriptions of typical Q-switched lasers can be found in U.S. Pat. No. 4,412,330 of Mauck et al. and U.S. Pat. No. 3,613,024 of Geusic et al.
A Q-switched laser may be employed, for example, to perform micromachining operations such as trimming thin-film resistors or repairing defective integrated circuits. The laser typically employs an acousto-optic Q-switch positioned between a pair of end mirrors in a laser cavity to control laser oscillation and thereby modulate the intensity of radiation within the laser cavity. By effectively blocking one of the end mirrors in the laser cavity, a Q-switch can remove the amount of cavity feedback and greatly increase cavity losses, thereby preventing laser oscillations. Thus when it is in a non-transmissive state, the Q-switch prevents output light emissions from occurring but allows continued storage of optical pumping energy in the laser gain medium. When it is in a transmissive state, the Q-switch increases the amount of cavity feedback and thereby greatly reduces cavity losses and allows the extraction of the stored energy in the form of an output light emission of relatively high peak power.
A Q-switched laser is typically employed in the micromachining of target structures having minimum critical dimensions of between 5 and 75 microns. Such micromachining operations have been successfully accomplished by a laser with output light beam position accuracy and shape tolerances of about 1.0 microns on the target structure. Operating a Q-switched laser within such tolerances could, however, cause the laser beam to machine incorrectly or miss a target structure having minimum critical dimensions of less than about 2.0 microns.